1. Field of the Invention
The present invention relates to a broad band high frequency (HF) or very high frequency (VHF) antenna of omnidirectional directivity and, more particularly, is directed towards a novel miniature wideband monopole antenna that has a load consisting of parallel conductors tuned to different resonant frequencies in which transmission line coupling occurs between adjacent conductors.
2. Description of the Prior Art
The purpose of an antenna is to convert the radio frequency (rf) circuit power provided at its input into output radiated power for transmission purposes and to convert radiated power into circuit power for reception purposes. Since the early days of radio, there has been constant endeavoring to improve the antenna, especially its characteristics pertaining to efficiency and bandwidth. The formula for the division of the rf power that is provided to an antenna is: EQU Input Power=Reflected Power+Dissipated Power+Radiated Power
Thus, for a fixed input power, decreasing the reflected power and decreasing the dissipated power will increase the radiated power (i.e., improve the antenna efficiency). Often wideband system requirements are limited by the antenna reflected power characteristics. Reflected power is caused by the impedance mismatch between the antenna input impedance characteristics and the characteristic impedance of the output stage or transmission line connecting the antenna to the transmitter. The difficulty of achieving a satisfactory impedance match over a broad band frequency range is caused by the antenna characteristics being frequency dependent and varying drastically with frequency. The frequency independent antenna is the only antenna class that minimizes broadband antenna characteristics changing drastically with frequency. However, frequency independent antenna theory and practice require physically large structures that approximate quarter wavelength (of lowest frequency) in two or more dimensions.
Certain applications require that a frequency independent VHF antenna be configured in one dimension, small in size, of light weight construction, storable for transportation and automatically deployable. One such application is as a deployable antenna from small fixed wing aircraft. Present broadband frequency independent antennas are inadequate for this application. Consequently, there is a need for a broadband HF-VHF antenna with all these characteristics.
Examples of prior art wideband antennas commonly use numerous monopole elements driven in parallel. These elements can also be driven with a 180 degree phase shift between elements as disclosed in U.S. Pat. No. 3,808,599 issued to Brunner. The monopole elements are usually tuned to separate frequencies within the operating band of the antenna array. This can be achieved by adjusting the length of the element or by adjusting a ferromagnetic core as disclosed in U.S. Pat. No. 3,931,625 issued to Chiron et al. However, none of these prior art antennas offer simultaneously the desired characteristics of a frequency independent, flexible, light weight, low mass, low air resistance, broadband antenna.